Cochlea: what is it, parts, functions and associated pathologies

Hearing, as the name suggests, is a term that encompasses the physiological processes that give human beings the ability to sense and relate to their surroundings on the basis of this essential sense.

In very general terms, the auditory process can be distinguished in the following events: the ear receives sound waves, which are transmitted through the ear canal to the eardrum, which produces a series of vibrations. These reach the bear chain, which is responsible for transmitting them to the inner ear through the oval window.

This is when it comes into play the cochlea or snail, an essential part of the mammalian hearing system. Immerse yourself with us in the world of auditory anatomy, because today we tell you what the cochlea is, its parts, the functions it performs and what happens when it fails.

    What is the cochlea?

    The cochlea is a spirally wound tube-like structure located in the inner ear, specifically in the temporal bone. Typically, this structure is around 34 millimeters long in an adult individual and, it should be noted, inside is the organ Corti.

    The organ Corti is essential for understanding the auditory process because it is made up of a series of sensory cells (around 16,000) arranged in a line, specifically called “hair cells”. They are the last ones responsible for “interpreting” the sound waves received by the outer ear, because they transform them into electrical impulses which reach the auditory nerve, and from there, the brain.

    Parts of the cochlea

    It is not yet time to describe the complex process involved in the integration of sounds in the brain, because we still have a lot of tissue to cut out in anatomical terrain. First of all, we can say that the cochlea is made up of three essential parts. We describe each of them:

    • Columella: with central which hosts the cochlear nerve.
    • Reticular blade: envelops the columella.
    • Spiral leaf: on which the inner wall of the reticular leaf rests.

    It should be noted that, beyond a description of the tissues observed in structural section, this gives us more information to look at the three longitudinal chambers that make up the cochlea. These are:

    • Tympanic ramp.
    • Vestibular ramp.
    • Average ramp.

    The tympanic ramp and the vestibular ramp contain perilymph (a serum fluid) and communicate with each other through a small duct called the helicotreme, located at the end of the cochlea. This allows communication and perilymphatic fluid between the two structures. For its part, the median ramp or cochlear canal is located between the vestibular ramp and the eardrum and contains the endolymph. This structure presents a fairly complex anatomy in terms of terminology, which is why we will limit ourselves to saying that it is triangular and that, finally, between the tympanic ramp and the median ramp is the organ Corti already named.

    Beyond this conglomerate, it should also be noted that these three chambers (tympanic, vestibular and median ramp) they are separated by two types of membranes: the Reissner membrane and the basilar membrane.

    The Reissner membrane separates the vestibular and medial ramp, and its function is to preserve the endolymph in the cochlear duct, where it must remain. On the other hand, the basilar membrane is responsible for the separation of the middle and tympanic ramps. Its function, however, is not so easily explained, as it relies on the organ of Corti. Let’s focus a little more on this very special membrane.

    The role of the basilar membrane in hearing

    First of all, it should be noted that the response of the basilar membrane to certain sounds will be affected by its mechanical properties, Which gradually vary from the base to the apex.

    At the end closest to the oval window and eardrum, this membrane has a more rigid, thick and narrow morphology. Therefore, its resonant frequency is high for high tones. On the other hand, at the distal end, the basilar membrane is wider, softer and more flexible, which improves low frequency response. Oddly enough, it can be said that this structure produces a ten thousand fold decrease in its stiffness from the proximal end to the distal end.

    At each point of this special membrane, a chord occurs, And where a greater displacement at a certain frequency takes place is called “characteristic frequency”. In other words, the range of resonant frequencies available in the basement membrane determines human hearing capacity, which is between 20Hz and 20,000Hz.

    Corti’s organ

    The basilar membrane analyzes frequencies, but it is the organ of Corti, the one responsible for decoding this information and sending it to the brain. We start from the beginning to understand how it works.

    We are again at the base of the inner ear: when a vibration is transmitted by the bones of the middle ear to the oval window, there is a pressure difference between the vestibular and tympanic cochlear cramps. As a result, the endolymph present in the midline shifts, producing a progressive wave that propagates along the basilar membrane.

    The movements of the basilar membrane move the hair cells (remember that they are the ones that make up the organ Corti) in relation to it. and, thanks to this, they are excited or inhibited depending on the direction of movement. Depending on which region of the basilar membrane oscillates the most in relation to the perceived sound, different parts of the hair cells that make up the organ Corti will be activated.

    Finally, hair cells produce certain chemical components that translate into nerve signals, which will first be sent to the acoustic nerve and then to the auditory nerve (also called cranial pair VIII). Of course, we are facing a very complex journey of understanding, but we can sum it up in the following concept: the basilar membrane “vibrates” more at one point or another depending on the type of sound, and the excited cells translate this. signal, which eventually reaches the brain through a series of nerves.

      What happens when the cochlea fails?

      It should be noted that hair cells do not regenerateIn other words, when they injure an individual, they irreparably lose hearing. We humans take our senses for granted until we lose them, which is why the World Health Organization (WHO) helps us contextualize a bit of what hearing loss means in general:

      • Over 460 million people worldwide suffer from hearing loss.
      • It is estimated that by 2050 this value will increase to 900 million, that is to say that one in 10 people will have a hearing loss.
      • 1.1 billion young people worldwide are at risk of hearing loss from exposure to excessive noise in recreational settings.

      Chronic exposure to loud sounds is an important factor that promotes hearing loss (hearing loss).. In these cases, the already described hair cells or the nerves that supply them are damaged at some point, causing the patient to hear the sound in a distorted way or, for example, to be easier to interpret the frequencies than to other.

      Finally, it is also essential to note that hearing loss due to age (presbycusis) is unfortunately quite normal. this process is observed in nearly 80% of seniors over 75 years old, And is caused by deterioration of structures located in the inner ear or the auditory nerve itself.

      summary

      As we have seen in these lines, the cochlea had many more secrets for us than we might imagine. From a complex morphology to the basilar membrane and the organ Corti, one concept is clear to us: hearing is a real work of engineering. Maybe all of this information makes us think twice the next time we turn the headphone volume up to the max, right?

      Bibliographical references:

      • What is the cochlea? Audifon, hearing centers. Collected November 12 at https://audifon.es/que-es/c/coclea-o-caracol/
      • Hearing and Cochlea, Medlineplus.gov. Retrieved November 12, from https://medlineplus.gov/spanish/ency/anatomyvideos/000063.htm
      • Cochlea, general: a journey into the world of hearing, cochlea.eu. Collected November 12 from http://www.cochlea.eu/es/coclea
      • Cochlea, vestib.org. Collected November 12 at https://www.vestib.org/es/coclea.html
      • Deafness, World Health Organization (WHO). Retrieved November 12, from https://www.who.int/es/news-room/fact-sheets/detail/deafness-and-hearing-loss
      • Soto, I., Vega, R., Chávez, H., and Ortega, A. (2003). Auditory physiology: the cochlea. Autonomous University of Puebla. Retrieved from: http: // www. physiology. BUAP. mx / online / DrSotoE / cochlea, 202,003.
      • Terreros, G., Wipe, B., León, A., and Délano, PH (2013). From the auditory cortex to the cochlea: Progression of the efferent auditory system. Journal of Otolaryngology and Head and Neck Surgery, 73 (2), 174-188.

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